The central objective of this proposal is to study the role of nucleoside pharmacology and host adaptive responses on HIV treatment outcomes. The interaction between host cellular factors and intracellular phosphorylation determine the efficacy or toxicity of these drugs. However, significant inter-individual differences exist in drug metabolism, toxicity and treatment efficacy among the nucleoside analogs. Potentially, intracellular drug interactions or suboptimal pharmacokinetics during nucleoside therapy may explain, in part, why as many as 30% of patients lose viral control with their first antiretroviral regimen. The project proposed in Aim 1 (CCTG 584) was developed to determine if a nucleoside interaction resulted in the high rates of early virologic failure that were observed in tenofovir, abacavir and lamivudine combinations. Although this raises concerns about the compatibility of this nucleoside combination, recent data suggest that when this regimen is combined with zidovudine high rates of treatment failure do not occur. At present it is unclear if the addition of zidovudine favorably alters HIV drug resistance pathways or enhances nucleoside metabolism of its companion analogs. The project proposed in Aim 2 (ACTG 5231 Sub-study) will evaluate the pharmacologic compatibility of this quadruple nucleoside regimen and the impact of specific combinations on intracellular nucleoside phosphorylation. An improved understanding of cellular nucleoside metabolism of combinations that result in treatment failure (CCTG 584) or treatment success (A5231s) will provide important insight as to how these agents can be optimally combined. To date, the intracellular pharmacokinetics of HIV drugs have been poorly studied in vivo, primarily due to methodological limitations. These projects will apply liquid chromatography-tandem mass spectroscopy to accurately measure intracellular drug levels and endogenous nucleotides in patient samples. Further, novel mechanisms of nucleoside drug resistance will be evaluated in these projects. Membrane transporters have an important role in the absorption, tissue distribution and cellular accumulation of nucleoside analogs. While it is widely recognized that the chemosensitivity of some tumors is correlated with the expression of certain transporters, our understanding of these drug transporters and their interaction with HIV drugs is still at an early stage. The goal of the candidate is to become a translational researcher capable of integrating laboratory techniques and new molecular models of HIV-drug therapy to answer contemporary clinical questions: The career development plan contains in-depth training in the responsible conduct of research, statistics, clinical trial design and laboratory-based techniques. The projects proposed in this application are novel and will address important questions in HIV therapeutics.